Diagnostic method for manufacturing processes

ABSTRACT

A method for use in a system for diagnosing the causes of manufacturing defects involves process characterization. A set of forms is identified for a workpiece and for a piece of manufacturing equipment that acts upon the workpiece. The forms for the workpiece are preferably a hierarchic set of geometric forms. Each such geometric form corresponds to an aspect of the action of the manufacturing equipment upon the workpiece. A plurality of measurements is made on a defective workpiece following the hierarchical order of forms. The measurements are compared to a reference datum, and a deviation from the datum is computed. If the deviation exceeds a preselected threshold, an alert condition results, attributable to the action of the manufacturing equipment. Targeted adjustment corresponding to the action that caused the defect can then be made to the equipment.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/585,423, filed Oct. 23, 2006, which is a continuation of U.S. patentapplication Ser. No. 11/322,688, filed Dec. 30, 2005, which is acontinuation of U.S. patent application Ser. No. 10/997,379, filed Nov.23, 2004, now U.S. Pat. No. 7,006,948, which is a continuation of U.S.patent application Ser. No. 10/236,450, filed Sep. 6, 2002, now U.S.Pat. No. 6,589,756. All of the foregoing applications are incorporatedherein by reference.

FIELD OF THE INVENTION

The invention relates to diagnostic analysis and, more particularly, tothe diagnosis of manufacturing defects.

BACKGROUND OF THE INVENTION

Manufacturing processes, and the products they create, sometimes sufferfrom defects. The job of detecting and eliminating these defects fallsto manufacturing engineers, who, over the years, have developedstatistical approaches in an attempt to address them. They have alsodeveloped various controls, ranging from process control to qualitycontrol to product control, for attempting to capture defective productsbefore they reach customers.

These controls are based on identification of limits on the capabilitiesof the various processes that are used in the manufacture of a workpieceor product. For example, process control is based on the capability ofthe process that produces the workpiece. An illustration is provided inFIGS. 1 a and 1 b, where shaft 10, produced on a lathe, has a nominaldiameter 20, and a sampling of shafts has a distribution of diameters 30at frequencies 40. The specified tolerance on the diameter dimension isidentified by reference numeral 60 as shown in FIG. 1 b. The range ofthe capability of the lathe is shown by numeral 50, where the capabilitymay exceed that of the range of the specified tolerance. Shafts may havediameters exceeding the upper specification level (USL) 70, while othersmay have diameters below that of the lower specification level (LSL) 80as shown in FIG. 1 b. Parts that exceed the specifications, according toa quality control approach, would be rejected.

So-called quality control procedures are directed to preventing“out-of-spec” parts reaching customers, but they do not necessarilyreduce the number of rejects. Rather, these approaches seek todistinguish good parts from bad based on product features that appear tobe readily measured, without revealing mechanisms responsible forproduct defects or physical insights that could more readily lead to thediscovery of such mechanisms.

SUMMARY OF THE INVENTION

The present invention provides approaches for identifying the causes ofmanufacturing problems by breaking down the different phases of amanufacturing process, such as the different actions of a machine toolon a workpiece, so that each phase or action can be related to theformation of an element or a feature of the final workpiece, preferablyproviding a one-to-one correspondence between the forming of the shapeof the feature and a process step or action that produces the shape.This procedure, then, helps reveal defects which process action producesit. Once the source of the defect is detected, both the source and theworkpiece defect it causes can be addressed.

The present invention provides not only a method for detecting defectsin a manufacturing process, but also a method for identifying where inthe process the defects occur, so that appropriate corrective action canbe identified, and such action taken, at the point of occurrence. Thisobject is accomplished by relying on a process characterization approachthat focuses on the actual effect of the process on a workpiece.

In one embodiment of an aspect of the present invention, a method foruse in a system for diagnosing the causes of deviation from an intendedform in a workpiece produced by a manufacturing process is provided. Atleast one form is defined for the workpiece and for a piece ofmanufacturing equipment that acts upon the workpiece to impart the form.A plurality of measurements for each workpiece is defined, each relativeto a respective reference datum. The subsequent steps involve generatinga record of the plurality of measurements corresponding to eachworkpiece and inferring from the comparison of the measurements for atleast one of the workpieces the existence of an alert conditionassociated with the action of the manufacturing equipment on theworkpiece.

Another embodiment of an aspect of the present invention involves amethod for identifying evidence of deviation from specification in aworkpiece produced by a manufacturing process, the manufacturing processbeing performed by respective manufacturing equipment. The methodincludes identifying a set of repeated portions of the workpiece, eachinstance of a repeated portion having forms, the form of one instance ofa repeated portion being substantially the same form as the otherinstances. For each instance of the repeated portion, a set ofmeasurements of the reproducible part is made relative to a respectivereference datum. Each set of measurements is compared to a respectivetarget range of values. Based on the comparison, the existence ofevidence of deviation from specification is inferred.

In still another embodiment of the present invention, a method forassessing a condition of a workpiece acted upon by manufacturingequipment starts by identifying a set of forms, each form correspondingto an aspect of the action of the manufacturing equipment upon theworkpiece. The subsequent steps involve making a plurality ofmeasurements for each form; computing, for each plurality ofmeasurements, a respective deviation from a corresponding referencedatum; defining a deviation threshold; and, if a computed deviationexceeds the deviation threshold, inferring the existence of thecondition attributable to the action of the manufacturing equipment onthe workpiece.

In yet another embodiment of an aspect of the present invention, amethod is provided for detecting deviations from an intended form in amechanical part. The deviations are detected on the basis ofmeasurements of geometric properties relative to a reference datum, thegeometric properties imparted by a machine tool operating on themechanical part. The method comprises the steps of: identifying ahierarchic set of geometric forms characterizing the mechanical part,each form corresponding to an action of the machine tool on the part;categorizing the geometric forms from a lowest order to a highest order;making a plurality of measurements corresponding to the lowest orderform; for each plurality of measurements, computing a respectivedeviation from a defining datum; checking for an alert condition foreach of the respective deviations; and if an alert condition is present,inferring a deviation from the intended form.

In another embodiment of an aspect of the present invention, a method isprovided for characterizing the ability of a machine tool to reproduce aproduct without deviating from a specification intended for at least aportion of the product, the characterization based upon taking geometricmeasurements. The method involves identifying a set of geometric formspresent in the product; selecting a first of the set of geometric forms;making a plurality of measurements corresponding to the selectedgeometric form; for each plurality of measurements, computing adeviation from a respective reference datum; checking for an alertcondition for each of the respective deviations; if an alert conditionis present, adjusting the machine tool and repeating the method from thestep of making additional measurements corresponding to the sameselected geometric form iteratively until no further alert condition isfound. If no alert condition is present, the method is repeated from thestep of selecting the next set of geometric forms by incrementing to thenext form until all geometric forms have been selected and no furtheralert condition is found.

The present invention also provides a method for representing measureddeviations for features attributable to the forming of a physicalobject, in a suitable frame of reference. The method involvesidentifying an order for the features; providing a first region forcomparing measurements corresponding to the features of the object;providing a second region associated with the first region; wherein thefirst region comprises frames of reference for the set of objects withrespect to which the measurements are represented; and wherein, in thesecond region, the order of the object features is represented incorrespondence with the measurements represented in the first region,and representing the measurements in a corresponding frames ofreference, in conjunction with respective representations of the orderof the measured objects.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a shows a conventional shaft.

FIG. 1 b is a frequency plot of a range of diameters of the shaft ofFIG. 1 a showing the range of capability of a lathe producing theshafts, and the range of tolerances, according to prior art.

FIG. 2 a is a top-view of an assembly of a shaft and its sleeve.

FIG. 2 b is a frequency plot of a range of diameters of the shaft ofFIG. 2 a showing an upper specification level and a lower specificationlevel.

FIG. 2 c is a cross-sectional view of the assembly of FIG. 2 a.

FIG. 2 d is a side view of the sleeve of FIG. 2 a.

FIG. 2 e is a side view of a defective shaft.

FIG. 2 f is an enlarged view of the defective shaft of FIG. 2 e showingvarious forms, in an embodiment of the present invention.

FIG. 3 a is an isometric view of a shaft showing a defectivecross-sectional area, according to the present invention.

FIG. 3 b shows a hierarchic arrangement of a category of geometricelements, or forms, in an embodiment of an aspect of the presentinvention.

FIG. 4 is a flow diagram showing an embodiment of a method according tothe present invention for diagnosing causes of defects in themanufacture of a shaft.

FIG. 5 is a flow diagram showing an embodiment of a method according tothe present invention for diagnosing causes of defects in amanufacturing process.

FIG. 6 a shows an arrangement of geometric forms and reproductive forms,in an embodiment of the present invention.

FIG. 6 b shows a shaft and its housing.

FIG. 7 a is an isometric view of an engine block deck.

FIG. 7 b is a graph of the distribution of measurements associated withsurface profile of the engine block deck of FIG. 7 a, in an embodimentof the present invention.

FIG. 7 c is an isometric view of the engine block deck of FIG. 7 ashowing a definition of geometric forms as points measured with respectto the vertical dimension of the block and line segments defined by aset of points according to the present invention.

FIG. 7 d is an isometric view of the engine block deck of FIG. 7 ashowing a definition of geometric forms as line elements, in anembodiment of the present invention.

FIG. 7 e is an isometric view of the engine block deck of FIG. 7 ashowing a definition of geometric forms in the form of pass elements inan embodiment of the present invention.

FIG. 7 f is a multi-form chart, comprising a combination of a multi-formgraph and a multi-form table showing the plot of the various elements,or geometric forms, that constitute the engine block deck of FIG. 7 a,in an embodiment of the present invention.

FIG. 7 g is a generalized multi-form chart of FIG. 7 h represented inpolar coordinates.

FIG. 8 a is a condensed chart showing a hierarchical arrangement of theforms of the engine block deck of FIG. 7 a.

FIG. 8 b shows the finding of an exemplary defect in one of the formsused in a graphic analysis of FIG. 7 f.

DETAILED DESCRIPTION

The present invention provides a diagnostic method for detecting defectsin a manufacturing process through an ability to control the process ofcreating shapes in a workpiece. A workpiece comprises elements, whichmay be portions and features, which are characterized by respectivegeometric forms. The method, broadly speaking, is accomplished throughcharacterization of the manufacturing process, specificallycharacterizing the manufacturing process as a plurality of aspects orsteps that correspond to actions involved in forming of the variouselements of a work product. Then, actions of the manufacturing processassociated with work product defects are identified. Where themanufacturing process involves forming a piece of hardware, an elementof a workpiece may be a portion or feature of the workpiececharacterized by a geometric form. Through a systematic and preferably(though not necessarily) hierarchically ordered set of measurements ofthe elements of an apparently defective product, one or more defectiveforms associated with the actions of the manufacturing equipment on theworkpiece can be detected. In this manner, the source of a defect may bemore efficiently and directly arrived at, and corrective action may morereadily be taken at the point of occurrence of the defect.

According to an aspect of the present invention, a method foridentifying geometric forms for a workpiece that is acted upon bymanufacturing equipment begins by characterizing the process steps usedin producing that workpiece or product. For this purpose, a flow diagramis constructed. Corresponding to each process step (that is,corresponding to each action of the machine tool) in the process flowdiagram, a corresponding physical form of an aspect of the workpiece isidentified and diagrammed. A practitioner may find it advantageous tobegin this exercise with the least complicated geometric form that,together with successively higher order forms, identify the overall therelevant geometry of the workpiece. A form may, for example, be a set ofpoints that define radii of a shaft, a set of radii that together definean arc of the shaft, a plurality of arcs that form circles (or closedcurves, at any rate), and the set of circles that define the shaft, or asegment of it. Any variance of the actual workpiece data correspondingto a form, relative to a limit imposed by a specification, also referredto here as a reference datum, may provide evidence of a correspondingworkpiece defect, the cause of which may be an aspect of the processstep associated with that workpiece form. The overall variance of actualmeasurements from a datum can be distributed equally between theselected process steps, or allocated according to the expected influenceof the forms on the functioning of the workpiece. However, no one formshould be allocated more than 50% of the variance.

Accordingly, the geometric forms are measured. They may include, withoutlimitation, radii, arc, closed curves (such as circles, ellipses, etc.)and surfaces. For each selected form, deviation of the allocatedvariance from the datum is computed. The form having the greatest degreeof deviation is postulated to be associated with a defect responsiblefor causing the unacceptable variance from the datum. A defect, if ofsufficient severity, may give rise to an alert condition. An alertconduction can be any condition recognized for the manufacturing processas one that may trigger an observation or other response from an entitywith responsibility for at least some aspect of the process. An alertcondition may be inferred, for example, if a deviation exceeds a definedthreshold based on a preselected rule. The rule may vary depending uponthe characteristics of the form. For example, for an arc, the rule maystate the limits of the angle that subtends the arc. Attention can thenbe focused on the errant process step causing the unacceptabledeviation, and an appropriate adjustment identified and implemented.Following one or more iterations of the adjustment process, the tendencyof the machine tool to generate the detected defects can be remedied.

Aspects of an embodiment of a method according to the present inventionare shown in FIGS. 2 a-2 f, in an example involving a malformedcylindrical shaft. FIG. 2 a shows a top view of an assembly of shaft 100in sleeve 110. The shaft and the sleeve are separated from one anotherby clearance 120. The nominal bore diameter of the sleeve is D 115,while the nominal diameter of the shaft is d 105. Acceptable ranges ofdiameters 105′ and 115′, within a set of specifications, are plotted inFIG. 2 b. A lower specification level (LSL) 103, and an upper levelspecification (USL) 107 are shown schematically in the same FIG. 2 b. Across-sectional view of the shaft and sleeve are also shown in FIG. 2 cfor clarity. The shaft has a base 109, and both the sleeve and the shafthave the same length l 125.

FIG. 2 d shows a side view of sleeve 110, with shaft 100 withdrawn fromthe sleeve. The shaft itself is shown in FIG. 2 e with its axis ofrevolution 104. FIG. 2 e also shows a second shaft superimposed on thefirst shaft. Second shaft 140 is “out of true” at its free end oppositethe base, that is, slightly out of shape as manufactured, thoughexaggerated in the drawing for illustrative purposes. Its axis ofrevolution is denoted as 144, and the shaft has a diameter d′ 145.Measured along their respective axis of revolution, the diameter of bothshafts, namely, d and d′ fall within the specified tolerances and theirlengths are the same. This is shown in FIG. 2 e by superimposing thedistribution of diameters of FIG. 2 b along the extended axis ofrevolution 144 of shaft 140.

Having met the upper limit and lower level specifications, both shafts,therefore, are expected to pass quality control. At the same time,however, it is evident from FIG. 2 e that, when shaft 140 is insertedinto sleeve 110, it is not expected to rotate freely, as the “bent,” ordefective, portion of the shaft will be contacting the inside wall ofthe sleeve.

FIG. 2 f shows an exploded isometric view of defective shaft 140.Examination of the shaft reveals that the radius form R 153 and the arcform S 155 in the plane of each of the circles C 150 are “true” withintheir respective specifications. Although all the forms, up to andincluding the circle form, meet their respective specifications, thecylinder form 140, generated by the repetitive action of a machine tool,such as a lathe, is out of true, its axis of revolution lying along 144shown in FIG. 2 f and not along axis 104. This condition requires acorrective action to adjust the action of the machine tool. Without acorrective action, the parts will not function properly, even thoughthey will meet the overall quality control specifications as depicted inFIGS. 2 b and 2 e.

A need for corrective action can arise at any one of the steps describedabove. FIG. 3 a shows another example of a defective shaft 300, a truecylinder with a straight axis of revolution 310, but also having aradius 301, which deviates beyond a threshold datum. Shown schematicallyin FIG. 3 a, R has a nominal value, and R1 and R2 deviate substantiallyfrom that datum. At this point, the machine is adjusted beforeproceeding further with the production of the workpiece. Once thiscorrective action is taken, then other forms can be examined in aniterative manner until the entire defect-causing process steps areremedied.

In one embodiment, shown in FIG. 3 b, geometric forms at each step aregrouped together such that the same forms at a given step, when takentogether constitute the next form, in a hierarchical fashion. Thus,radii 301, constitute the next higher order arc form 302. All radii 301inscribe the same arc 302. Then, arcs taken together constitute circles,and circles together define a cylinder or shaft. The forms are shapeelements, which together constitute more complex shapes. It is thedifference, or deviation Δ, of the measured values of the forms from areference datum at a given step that triggers an alert condition towhich attention must be given. Accordingly, the process stepcorresponding to the form whose deviation gives rise to the alertcondition that must be adjusted to eliminate that alert condition.

An embodiment of a method according to the present invention applied tothe fabrication of a mechanical shaft is shown in the flow diagram ofFIG. 4. A set of forms for a shaft in the same Figure is identified instep 400. The forms are the radius, arc, circle and cylinder. The datafrom which the forms are generated are identified 405. Then the form isset to that which is lowest in hierarchy 407, and the form is selected410 (radius, in this case). Starting with the first selected form, threeradii are measured at random (steps 420-440). A characteristic value,such as an average, μ, of the three radii, is next calculated in step450. Then, an absolute value of the difference between thecharacteristic value, μ_(avg.) and a given reference datum, orμ_(datum), is calculated. In step 460, the deviation of thecharacteristic value from the datum is compared with a threshold value.If the deviation is greater than the threshold value, then an alertcondition 470 is declared. Consequently, the process step correspondingto the selected form, is adjusted at 480 and a new set of radiimeasurements is made (following steps 420-470) in an iterative fashionuntil alert condition is remedied.

When there is no alert condition, a second set of forms is selected atstep 490 by incrementing to next form in hierarchy 495, and returning tostep 410. In the example shown in FIG. 4, three measurements are madefor the arc form, S. Then, steps 420-460 are repeated. If the deviationfor the arc measurements exceeds a threshold value, step 470, then theaction of the machine corresponding to the production of the form isadjusted accordingly at step 480. The process is continued iterativelyfor circle and cylinder forms until measurements for all forms arecompleted, any alert condition remedied, and until no further adjustmentof the process machine is needed 500. At step 510, the machine is readyto produce forms according to a shop print, and, consequently, theworkpiece should be defect-free.

FIG. 5, which shows steps 600-710, is a generalization of the processsteps of FIG. 4, where there are N forms that make up a workpiece, or aproduct. A category of geometric form 600 comprises, in general, acurvilinear segment, a closed curve, or a surface. Another category cancomprise spatial forms or temporal forms. The latter forms representreproductive elements within which at least portions comprisinggeometric forms of a workpiece can be reproduced repeatedly andfaithfully from location to location on a factory floor, or over a timeinterval of hours, days, or weeks.

In similar steps of FIG. 4, data from which the forms are generated areidentified 605. Then the form is set to that which is lowest inhierarchy 607, and the form is selected 610. Starting with the first ofthe N forms, first measurement of first set of forms is performed 620.Second and third measurements are performed on the first set of forms630 and 640, respectively. A characteristic value is next calculated instep 650. Then, the difference between the characteristic value and agiven reference, or datum, is calculated. In step 660, the deviation ofthe characteristic value from the datum is compared with a thresholdvalue. If the deviation is greater than the threshold value, then analert condition 670 is declared. Consequently, the process stepcorresponding to the selected form, is adjusted at 680 and a new set ofmeasurements is made (following steps 620-670) in an iterative fashionuntil alert condition is remedied.

When there is no alert condition, a second set of forms is selected atstep 690 by incrementing to next form in hierarchy 695, and returning tostep 610. Steps 620-660 are repeated. If the deviation for themeasurements exceeds a threshold value, step 670, then the action of themachine corresponding to the production of the form is adjustedaccordingly at step 680. The process is continued iteratively for theremaining forms until measurements for all forms are completed, anyalert condition remedied, and until no further adjustment of the processmachine is needed 700. At step 710, the machine is ready to produceforms according to a shop print, and, consequently, the workpiece shouldbe defect-free.

FIG. 6 a shows a recast of the geometric forms 830 of FIG. 3 b forillustrative purposes. Geometric elements, or forms, in the form ofshapes 850, form portions of, and the workpiece itself, such as theshaft shown in FIG. 6 b. Reproductive forms, such as the locations wherethe shafts are made and the time intervals within which they are made,are all identified and assessed for alert conditions. The correspondingacts related to the manufacturing process are corrected to produce formsin accordance with a specified print to result in a defect free product.In order to achieve the reproducibility of a multiplicity of the sameshaft in a period of time 840, the same analysis as shown in FIG. 5 isextended temporally to account for deviations from a datum where Nincludes time as a form.

In one embodiment, a method for identifying defects through a processcharacterization approach according to the present invention isdescribed with reference to an example involving an automobile engineblock deck 900, shown in FIGS. 7 a-7 h. The deck constitutes one-half ofan engine block. After the other half (not shown) is sealed and boltedon, the engine block is readied for operation. As detected duringoperation, the engine block leaks oil. The method according to thepresent invention is applied to find the defect, so that the necessaryadjustments to the machine tool that produced the engine block scan beappropriately adjusted to produce engine blocks that are defect free anddo not leak oil.

A specified flatness profile for the deck surface 910 shown in FIG. 7 ais plotted as graph 930 in FIG. 7 b. The existing flatness profile ofthe deck is as shown in graph 920 of the same Figure. The deviation ofthe existing surface profile from the required surface profile wellexceeds the upper speciation level (USL) 933 and the lower specificationlevel (LSL) 935 as shown in the same FIG. 7 b. In order to identify theprocess step or steps causing the defective surface profile, the variousforms that constitute the deck are measured in a hierarchical order. Themeasurements are then subjected to a multi-form analysis using amulti-form deviation chart. A multi-form deviation chart of the presentinvention uses a multi-vari graph with additional features that will befurther described below

An examination of the block deck surface reveals that the simplestmeasurable forms that constitute the surface comprise: 1) a point on thesurface, 2) line segments defined by a plurality of points, and (3)lines defined by line segments. Points 940 and line segments 950 areshown in FIG. 7 c. Lines 960 are shown in FIG. 7 d, where at least twolines form a planar surface 910. The planar surface of the deck isformed by one pass of the machine tool in a single direction 970, and byanother pass 970 in the other direction, as shown in FIG. 7 e. Oncehaving identified the forms to be measured, the measurements are madeaccordingly. The measurements are then represented in a multi-formdeviation chart as shown in FIG. 7 f. The multi-form deviation chartcomprises a combination of a multi-form graph 980 and a multi-form table985 as shown in the same Figure.

In the illustrated embodiment shown in FIG. 7 f, the multi-formdeviation chart comprises a multivari plot of the data. Three datapoints are preferred for each of the selected forms. The measured valuesare plotted in a multi-form graph 980. However, before making themeasurements, it is preferable that the selected forms are firsttabulated in multi-form table 985 presented with the multi-form graph asshown in FIG. 7 f. The spatial relationship between multi-form graph 980and multi-form table can be contiguous or proximally adjacent. Thus,starting with one deck, two pass measurements are needed, since, atminimum, two passes are required to form one deck surface. Three linesconstitute one pass so that three lines are measured for each pass.Finally, three line segments are measured for each line. Lowest ordergeometrical form “points” 940 define the next higher order geometricalform “line segment” 950. However, in defining geometric form “line” 960,“line segments” 950 are lower order forms with respect to 960. Likewise,form “pass” 970 is constituted by three “line” forms. That is, pass 970occupies a higher order in the form hierarchy.

For multi-form analysis of additional engine block decks, the multi-formtable and the accompanying multi-form graph are extended laterally L,and the measurements are repeated in exactly the same manner asdescribed above. A continuation for a second deck is shown in FIG. 7 f.Furthermore, for multi-form analysis of additional forms, such as formonitoring variations in decks manufactured at differentstations/locations on the same manufacturing floor, which may becharacterized as spatial variations, or for monitoring temporalvariations in decks manufactured in certain periods of time, such as agiven day, or week, the multi-form table can be extended vertically V toaccommodate the additional forms.

As shown in FIG. 7 f, multi-form graph 980 and multi-form table 985 arepresented together to form a multi-form deviation chart. Preferably,deviations from a reference datum are represented by the average of themeasured values in each category of form, although other mathematicalrelationships can be employed. FIG. 7 f is constructed using Cartesiancoordinates. Other frames of reference can also be used. Thus, in FIG. 7g, polar coordinates are used. In the same Figure, reference numerals900′, 940′, 950′, 960′, 970′ and 980′ correspond to the same unprimedreference numerals of FIG. 7 f. The space inside the solid boundariesrepresents the multi-form graph region, and the dashed space outside thesolid boundaries represents the region for placement of ordered forms.Additional ordered forms may be accommodated by expanding the polarspace outwardly, O. A condensed table having additional forms is shownin FIG. 8 a. Lower order forms 980 are shown in boxes 990, while higherorder forms 995 are shown below those boxes.

An analysis of the data plotted in a multi-form deviation chart makes itpossible to identify form(s) having the greatest variance, that is, withthe accompanying alert condition. The methods of the present inventionshow that non-random alert conditions generally are most likely toappear, if at all, in geometrical forms. On the other hand, random alertconditions, when they occur, are mostly found in reproductive forms,such as in reproduced workpieces themselves, or in spatial or temporalforms. Accordingly, the corresponding process step(s) or action(s) ofthe manufacturing equipment is (are) adjusted. This is shown in FIG. 8b, where for illustrative purposes, a profile defect of about 0.48 mm isfound in a line form comprising lower order line segment forms.Consequently, the process step (the action of a milling machine, forexample) responsible for milling lines/line segments is adjustedaccordingly so that the ensuing engine blocks are free of the previouslydetected defect, and no longer leak oil.

While the invention has been shown and described with reference toparticular embodiments, those skilled in the art will understand thatvarious changes in form and details of the methods according to thepresent invention may be made without departing form the spirit andscope of the invention.

1. A method for diagnosing a cause of deviation from an intended form ina workpiece produced by a manufacturing process, in which at least oneform is defined for the workpiece and manufacturing equipment acts uponthe workpiece according to the manufacturing process to impart the form,the method comprising the steps of: defining a plurality of measurementsfor the workpiece, each relative to a respective reference datum; takingthe defined measurements for the workpiece; generating a record of themeasurements; comparing the recorded measurements for each workpiece;and inferring from the comparison of the measurements the existence ofan alert condition associated with the action of the manufacturingequipment on the workpiece.